Browsing by Author "Kratz, Rolf Dietmar"

Now showing 1 - 6 of 6
  • Thumbnail Image
    Item
    Open Access
    Control of cracking in concrete by steel reinforcement : an examination of some of the mechanisms involved
    (1986) Wilson, Derek Leslie Skeeles; Kratz, Rolf Dietmar
    The techniques tried up till now to model the problem of cracking in reinforced concrete have been largely empirical. Many relationships have been identified between certain parameters and the occurrence and size of cracks. This thesis reviews these, reports on the current state of knowledge, and discusses some of the theories proposed. Because of the great variation and complexity of the materials and stress interactions involved, the problem has to date defied completely objective theoretical modelling. The finite element method provides a powerful new modelling tool for theoretical simulation of complex real problems. Recent developments on constitutive models for concrete make this method extremely attractive for use in this case. This thesis attempts to make use of these tools by carrying out some analyses of the cracking of reinforced concrete prisms stressed in tension. The method is found to be a viable way of examining mechanisms and effects which are not ordinarily visible in laboratory experiments. These mechanisms are discussed in the light of the experimental findings recorded in the literature.
  • Thumbnail Image
    Item
    Open Access
    The effects of impact loading on reinforced concrete panels
    (1992) Mason, Derek Allen; Kratz, Rolf Dietmar
    An investigation into the effects of impact loading on reinforced concrete panels (slabs) was conducted. The impact load was generated by means of a compound pendulum. The impact load was increased by increasing the height from which the pendulum was released as well as by the addition of weights (masses) to the pendulum. The duration of the impulse was varied by the addition of weights (masses). This meant that with low mass and high velocity a high initial intensity and short duration was achieved. The addition of weights (masses) gave low initial intensity and longer duration. Damage criteria identified included residual strength, pendulum backswing, crack width and permanent deformation as well as 'punching' through of the panels. It was shown that residual strength was inadequate on its own to classify a specimen as failed, but had to be used in conjunction with one or more of the other damage criteria. An Elasto-Plastic Design Method was developed and used in the analysis. The method is described in Chapter 8 and is given as a simplified method to design panels for impact loading.
  • Thumbnail Image
    Item
    Open Access
    Effects of reinforcement corrosion on the structural performance of reinforced concrete beams
    (1994) Theron, Gavin De Vos; Alexander, Mark Gavin; Kratz, Rolf Dietmar
    This dissertation is an investigation into the effect of reinforcement corrosion on the structural performance of reinforced concrete beams. Two types of specimens are investigated, the first without any stirrups and the second with stirrups. The specimens were corroded galvanostatically as well as by subjecting them to alternate cycles of wetting and drying with a saline water. An attempt is made at classifying the extent of corrosion of the reinforcing steel and its effects on the concrete. The effect of the corrosion on the structural performance is measured by establishing its effect on the maximum load carrying capacity, the deflections, energy requirements and ductility ratio. The main conclusions made in respect of the effect of reinforcement corrosion are that it causes: a decrease in the load carrying capacity; an increase in the deflections at the equivalent load level; a decrease in the energy requirements to reach the maximum load; and a smoothing of the load-deflection relationship. A limited literature review is also presented to provide background information of corrosion in concrete and general structural behaviour. Guidelines for the development of an analytical model to predict the load carrying capacity of corrosion affected reinforced concrete beams are also given.
  • Thumbnail Image
    Item
    Open Access
    An experimental investigation into the effectiveness of steel fibre-reinforced concrete relative to conventional mesh-reinforced concrete in thin shell construction
    (1988) Neumann, Michael J E; Kratz, Rolf Dietmar
    This report presents the results of an investigation into the suitability of steel-fibre reinforced concrete (SFRC) as a building material. The performance of the SFRC is gauged relative to conventional steel mesh-reinforced concrete. The report begins with the historical background of fibre-reinforcing, its development and diversity in modern times. The theoretical principles governing the strength of SFRC are formulated, followed by an extensive review of factors influencing the fibre's potential and its subsequent effect on the mechanical properties of a concrete matrix. The experimental program is thereafter detailed in two stages: the "Efficiency and Proving Phase" followed by the "Application Phase". The former involves a series of tests to determine the most promising fibre of the five types available, while the latter compares the strength properties of slabs and arches reinforced with this fibre to those reinforced with wire mesh. The test results obtained from the two phases are evaluated and discussed in the succeeding two sections and the report ends with the conclusions drawn and recommendations regarding the effectiveness of SFRC relative to mesh-reinforced concrete.
  • Thumbnail Image
    Item
    Open Access
    An investigation of concrete curing practice in the Cape Town area
    (1995) Krook, Andre; Alexander, Mark Gavin; Kratz, Rolf Dietmar
    [Watermark not visible] This project describes the results from the use of durability index tests as a tool for the investigation of the concrete curing practice in the Cape Town area. The object of the tests was to determine the effect environmental conditions have on the physical properties of the outer skin of concrete. The laboratory work involved the exposure of three concrete strengths to various relative humidities. The site work involved testing the cured outer surface of concrete at 28 days on six construction sites. The oxygen permeability and water sorptivity tests were used to investigate the outer surface of concrete. The laboratory work showed that curing at a relative humidity of 90 had a beneficial effect on the durability index values, while curing at 60 relative humidity was less effective. The permeability and sorptivity of the site results remained constant as the actual strength increased. Furthermore, permeability and sorptivity increased as the average evaporation rate increased.
  • Thumbnail Image
    Item
    Open Access
    A new ultimate limit state approach to the design of prestressed concrete beams
    (1990) Hallam, Grant Robert; Kratz, Rolf Dietmar
    The present approach to the design of prestressed beams is antiquated and time consuming. Neither SLS or ULS requirements are satisfied directly. There is a need for a new approach using plastic principles to design the prestressing requirements at ULS considering a whole span at a time, with checks made for SLS requirements afterwards. For a plastic design, the designer would need to know the limits of the bending moment redistribution for the beam under consideration. An equation is therefore necessary to assist the designer in this regard. Such an equation should take into account the cross-section shape of the beam and the prestress to reinforcing steel ratios. Many examples would have to be investigated using a rigorous plastic analysis to formulate such an equation. A computer program has been written as part of this thesis to perform such a rigorous analysis. It's accuracy has been evaluated by comparison with laboratory test beam results. The comparison was favourable, although more results would have to be compared to establish the accuracy that could be expected. A few examples were investigated and the observations noted. Two extreme cases were examined, those of a T-beam and I-beam. The redistribution of bending moments was similar in both cases, but the percentage of the central moment capacity reached was considerably less for the T-beam. For both beams it was impossible to reach the ultimate capacity at the centre before the strain capacity at the supports was exhausted. It was concluded that a plastic design procedure should be introduced, based on the ULS capacity of a span at a time. SLS requirements could then be checked afterwards. This approach would provide a design procedure corresponding to those used for other forms of modern structural design and would be much quicker, safer, more accurate and less costly that the existing design procedures.